Production of cellulose triesters



United States Patent PRODUCTIGN OF CELLULOSE TREESTERS JohnlP. Stoneman, Coventry, England, assignor to Courtaulds Limited, London, England, a British company Application December 3, 1951, Serial No. 259,705

' N0 Drawing.

This invention relates to the production of cellulose triesters.

. It' is known (see for example United States patent specification No. 1,861,320) that cellulose in fibrous form may be esterified whilst maintaining its fibrous structure by treating the fibres with an esterifying solution comprising a fatty acid anhydride, an esterification catalyst and an inert solvent or solvent mixture, that is to say'a solvent or solvent mixture which is miscible with the anhydride and the catalyst but which will not dissolve cellulose or the cellulose triester. I

It has also been proposed, for example in French patent specification No. 929,968, to produce cellulose esters in fibrous form by treating a stationary cellulose mass with an esterifying liquid which is passed through the mass of cellulose.

The cellulose hitherto employed in the fibrous esterification processes using a static bed of cellulose has generally been in the form of opened wood pulp or cotton linters. Such forms of cellulose, after activation for example with a fatty acid, have open and disintegrated structures which offer a large resistance to the flow of the esterification liquid; there is consequently a maximum depth of bed beyond which it is difiicult to maintain the required rate of liquid flow and as a result it is difficult to control the heat of reaction particularly in the early stages. In addition to the main acetylation reaction there is believed to be an immediate exothermic side reaction in which the water present as moisture in the cellulose reacts with acid anhydride in the esterifying liquid. If the depth of the cellulose bed is excessive, the acetylation reaction starts before the water-anhydride reaction is finished and consequently it is diificult to remove the heat evolved and poor products of variable viscosity and homogeneity are obtained.

It has been proposed in United States patent specification No. 2,087,036 to esterify cellulose in continuous sheet form, but such a form of cellulose is unsuitable for use as a static bed as the esterifying liquid tends to flow round the edges of the sheets rather than through the sheets and consequently acetylation is uneven.

The object of the present invention is economically to esterify static beds of cellulose in batches larger than used hitherto.

According to the present invention, in a process for the production of cellulose triesters by passing an esterifying liquid comprising a fatty acid anhydride, an esterification catalyst and an inert solvent or solvent mixture through a static bed of cellulose under such conditions that the fibrous form of the cellulosic material is maintained, the cellulose is used in the form of substantially flat wood pulp chips, that is to say, square or rectangular pieces of wood pulp sheet having a thickness of between 0.05 and 0.20 cm., a length and breadth of not more than 9 cm. and a bulk density within the range of 0.3 to 0.8 gram per cubic centimetre. The chips preferably have a thickness of between 0.07 and 0.12 cm. and a bulk density of from 0.4 to 0.5 gram per cubic centimetre.

2,721,865 Patented Oct. 25, .1955- The term bulk density as used herein means the weight in grams of 1 cubic centimetre of the wood-pulp sheet;

The wood pulp chips are pretreated with the appropri ate fatty acid before or after the bed of cellulose is formed in the reactor. The pretreatment is preferably effected by steeping the chips in glacial acetic acid and then removing excess acid by draining, the usual pretreated chips containing about 2 parts by weight of acid to 1 part by weight of chips. Opened wood pulp and cotton linters require pressing or centrifugingin order to remove excess acid but such processes are unnecessary using wood pulp, draining being'suflicient to leave the required amount of acid on the chips.

Wood pulp chips as used in the present invention have a higher bulk density than opened Wood pulp and cotton linters and consequently a given reactor can house more cellulose material in the form of wood pulp chips; in addi* tion the resistance to flow of the chips is lower and therefore a deeper static bed or larger plant may be used. The chips areas chemically reactive as the other forms of cellulose but because of their physical form, the liquid takes longer to penetrate the-chips, the heat of reaction is given out more slowly and heat control is correspondingly simplified. The effect of wood pulp chips in increasing the batch size of a plant may be illustrated by reference to specific reactors. With a reactor 3 feet deep and having a volume of 2.36 cubic feet, wood pulp chips can be satisfactorily acetlylated when packed at 8.4 pounds per cubic foot whereas opened wood pulp can only be packed at a maximum of 4.2 pounds per cubic foot and cotton linters at a maximum of 5.6 pounds per cubic foot to give satisfactory acetylation. With a reactor 3 feet deep and having a volume of approximately cubic feet, wood pulp chips can be satisfactorily acetylated when packed at 9.8 lbs. per cubic foot.

The present invention is illustrated by the following example in which parts and percentages are by weight:

Example 1 part of wood pulp sheets (bulk density 0.48) containing approximately 7 per cent of moisture were cut into pieces measuring approximately 2.5 cm. by 1.2 cm. The chips obtained were soaked for 8 hours in 7 parts of glacial acetic acid at 35 centigrade and after removal from the acid the chips were allowed to drain so that they retained approximately twice their own weight of acid. The pretreated chips were then packed randomly in a reactor 3 feet deep and an acetylation liquor containing 4.5 parts'of acetic anhydride (95 per cent), 25 parts of carbon tetrachloride, 0.003 part of 60 per cent perchloric acid and 0.022 part of sulphuric acid was pumped through with a linear velocity of 1 foot per minute. At the start the temperature was 20 centigrade and after minutes it had risen to 35 centigrade at which temperature it was maintained 210 minutes. The acetylation was then stopped by adding sufiicient sodium acetate to neutralise the acid catalysts. The fibrous product was then steamed and washed in the reactor and was finally dried.

The product was a fibrous cellulose triacetate having a 62.1 per cent acetyl content calculated as acetic acid; it dissolved in a mixture of 9 parts methylene chloride and 1 part ethyl alcohol to form a clear fibre-free solution suitable for spinning into fibres or casting into films.

What I claim is:

1. A process for the production of cellulose triacetate comprising the steps of soaking substantially smooth fiat wood pulp chips having a thickness of between 0.05 and 0.20 cm., a length and breadth of not more than 9 cm., and a bulk density within the range of 0.3 to 0.8 gram per cubic centimetre in acetic acid, thereby activating the cellulose in said chips, draining away excess acid, us-

ing said soaked chips as a static bed of activated cellulose, passing a substantially anhydrous acetylating liquid comprising acetic anhydride, an esterification catalyst and at least one solvent which is miscible with said anhydride and said catalyst, but inert to cellulose and cellulose acetate, through said static bed while maintaining the fibrous form of the cellulosic material, and continuing to pass theacetylation liquid through the bed until the cellulose is converted substantially into cellulose triacetate.

, 2. A process for the production of cellulose triacetate comprising the steps of soaking substantially smooth flat wood pulp chips having a thickness of between 0.07 and 0.12 crnt, a length and breadth of not more than 9 cm., and a bulk density within the range of 0.4 to 0.5 gram per cubic centimetre in acetic acid, thereby activating the cellulose in said chips, draining away excess acid, using said soaked chips as a static bed of activated cellulose, pa sing a substantially anhydrous acetylating liquid comprising acetic anhydride, an esterification catalyst and at leastone solvent which is miscible with said anhydride andsaid catalyst but inert to cellulose and cellulose acetate through said static bed while maintaining the fibrous form of the cellulosic material and continuing to pass the acetylation liquid through the bed until the cellulose is converted substantially into cellulose triacetate.

3. A process for the production of cellulose triacetate comprising the steps of soaking substantially smooth flat wood pulp chips having a thickness of between 0.05 and 0.20 cm., a length and breadth of not more than 9 cm., and a bulk density within the range of 0.3 to 0.8 gram per cubic centimetre in acetic acid, thereby activating the cellulose in said chips, draining away excess acid so that the chips retain approximately twice their weight of acid, packing said chips in a reactor as a static bed of activated cellulose, passing a substantially anhydrous esterifying liquid comprising acetic anhydride, an esterification catalyst, and at least one solvent which is miscible with said anhydride and said catalyst but inert to cellulose and cellulose acetate, through said static bed while maintaining the fibrous form of the cellulosic material, and continuing to pass the acetylation liquid through the bed until the cellulose is converted substantially into cellulose triacetate.

References Cited in the file of this patent UNITED STATES PATENTS Re. 21,935 Goff et al. Nov. 4, 1941 1,851,008 Hanson Mar. 29, 1932 1,861,320 Rheiner May 31, 1932 2,028,080 Stern Ian. 14, 1936 2,039,290 Berl May 5, 1936 2,074,339 Miles Mar. 23, 1937 2,087,036 Malm et al. July 13, 1937 FOREIGN PATENTS 929,968 France Jan. 13, 1948 

1. A PROCESS FOR THE PRODUCTION OF CELLULOSE TRIACETATE COMPRISING THE STEPS OF SOAKING SUBSTANTIALLY SMOOTH FLAT WOOD PULP CHIPS HAVING A THICKNESS OF BETWEEN 0.05 AND 0.20 CM., A LENGTH AND BREADTH OF NOT MORE THAN 9 CM., AND A BULK DENSITY WITHIN THE RANGE OF 0.3 TO 0.8 GRAM PER CUBIC CENTIMETRE IN ACETIC ACID, THEREBY ACTIVATING THE CELLULOSE IN SAID CHIPS, DRAINING AWAY EXCESS ACID, USING SAID SOAKED CHIPS AS A STATIC BED OF ACTIVATED CELLULOSE, PASSING A SUBSTANTIALLY ANHYDROUS ACETYLATING LIQUID COMPRISING ACETIC ANHYDRIDE, AN ESTERIFICATION CATALYST AND AT LEAST ONE SOLVENT WHICH IS MISCIBLE WITH SAID ANHYDRIDE AND SAID CATALYST, BUT INERT TO CELLULOSE AND CELLULOSE ACETATE, THROUGH SAID STATIC BED WHILE MAINTAINING THE FIBROUS FORM OF THE CELLULOSIC MATERIAL, AND CONTINUING TO PASS THE ACETYLATION LIQUID THROUGH THE BED UNTIL THE CELLULOSE IS CONVERTED SUBSTANTIALLY INTO CELLULOSE TRIACETATE. 